Apparatus for roasting ores



INVENTORS 4 Sheets-Sheet 1 E. KLEPETKO ETAL APPARATUS FOR ROASTING ORES July 3, 1951 Filed Aug. 18, 1948 y 3, 1951 E. KLEPETKO EIAL 2,558,963

APPARATUS FOR ROASI'ING ORES Filed Aug. 18. 1948 4 Sheets-Sheet 2 FIG. 2

NV NTO s Ear/rea i ned 761740 cred/ eye ATTORNEYS y 1951 E. KLEP ETKO ETAL 2,558,963

APPARATUS FOR ROASTISIG QRES Filed Aug. 18, 1948 4 Sheets-Sheet 3 mv: TO 5 t 17' 95970 -%77 e BY c2 yzzwm ATTORNEYS y 3, 1951 I E. KLEPETKO ETAL 2,558,963

APPARATUS FOR ROASTING ORES Filed Aug. 18, 1948 4 Sheets-Sheet 4 INVENTORS Erna 1 K1: etko gYbilp ab )Yey ATTORNEYS Patented July 3, 1951 ICE APPARATUS FOR ROASTIN G ORES Ernest Klepetko, Bauer, and Philip de B. Kaye, Salt Lake City, Utah, assignors to Combined Metals Reduction Company, Stockton, Utah,-a

corporation of Utah Application August 18, 1948, Serial No. 44,866

8 Claims. 1

This invention relates to roasting ores, concentrates, and like metallurgical products, and has for its principal object the provision of new and improved roasting apparatus. The new roasting apparatus is more or less of the multiple hearth type, but is characterized by having a plurality of laterally spaced hearth segments, substantially of sector shape, in the plane of each hearth level, rather than a single circular hearth, and is designed to effect an interrupted flash roasting of the crushed ores or concentrates with which it is charged. This interrupted flash roasting involves causing the particles of charge to fall freely in a stream of low particle density in the roasting chamber atmosphere, but only for a small fraction of the height of the chamber in any one uninterrupted drop. Each period of free fall of the charge particles is interrupted for a length of time that may be varied to suit individual conditions, but which generally does not exceed about one-quarter hour, and which in any event is substantially less than the time during which charge particles are held on any given hearth of a conventional multiple hearth roaster.

It is the purpose .of the invention to combine the advantageous features of flash roasting, or

suspension roasting as it is sometimes called,

. with the benefits of the bed roasting that proceeds on the hearths of conventional multiple hearth furnaces, and at the same time to avoid to the greatest extent possible the disadvantages inherent in apparatus for carrying out either of these methods separately. Flash roasting involves causing a finely divided charge of oxidizable ore or concentrate to fall freel through a heated oxidizing atmosphere in a suitable chamber. Roasting time is very short, being substantially no longer than the time required for the charge to fall the height of the chamber. The particles of charge must be very finely divided or they will be incompletely roasted in this short period. Multiple hearth furnace roasting, on the other hand, involves heating a bed of the charge on each hearth of the furnace in contact with the usually oxidizng furnace atmosphere.

The charge is continually turned over by rakes or rabbles to expose fresh unoxidized portions to the atmosphere, and is slowly advanced across each hearth and from hearth to hearth by the action of the rabbles.

Flash roasting has the advantages of greater roasting capacity per unit installation cost, elimination (in most cases) of external fuel requirements, and production of roaster gases rich in sulphur dioxide and well suited to sulphuric acid manufacture. These advantages stem from the very short period of intensive oxidation to which each particle of the charge is subjected. Great though these advantagcs are, they have not enabled flash roasting to supplant bed roasting in multiple hearth furnaces, because of the greater flexibility and ease of control of the latter type of roasting. Flash roasting requires a much more finely divided and more carefully dried charge than hearth roasting, and generally results in production of undesirably large quantities of dust in the flue gases (especially when the flash roaster is strongly up-drafted to hinder the free fall of the particles and thus increase the time allowed for roasting to proceed to completion). Control of the roasting temperature is exceedingly difficult in flash roasting operations, so that control of the chemistry of the roasting process is correspondingly diflicult. The flue gases themselves commonly are at such a high temperature as to make it difficult to handle them. Because multiple hearth roasting is not subject to these disadvantages it has heretofore been favored for most metallurgical roasting operations.

Variousproposals have been made heretofore to achieve some of the advantages of flash roasting in ordinary multiple hearth roasting operations. For example, it has been recognized that a very substantial part of the sulphur is burned from a charge of sulphide ore or' I concentrate in a multiple hearth roaster during the time that the material drops from one hearth to the hearth next below. To capitalize on this factor, the number of drop holes on the fout hearths of multiple hearth furnaces have been increased from 6 per hearth on early Evans- Klepetko furnaces to 16 per hearth on furnaces built in the last decade. Another proposal, used with some success, has been to remove the central hearths from a conventional .multiple hearth furnace, so as to form a flash roasting chamber between the top few and bottom few hearths. To the extent that these expedients have attained the advantages of flash roasting, however, they have resulted in introducing to a corresponding extent the disadvantages of the method. Very flnely divided and well dried charges have been necessary for optimum results, and control of the flash roasting temperature isnot achieved to any very great degree.

In the course of an exhaustive study of roasting problems, we have sought to balance ideal-' ized roasting conditions against the practical requirements of economical commercial practice, and have arrived at a set of criteria which has served as the basis on which we have developed our new roasting apparatus. The dominant elements of the criteria by which We have been guided may be summarized as follows:

1. Maximum use should be made of suspension roasting, within'the limits imposed by the vdiscontinue furnace operation.

The roasting which the new roasting apparatus so more rapidly roasted particles to pass through the roasting operation with the least possible delay.

3. An adequate supply of air or other oxidizing gas should be available wherever roasting is proceeding most rapidly, and especially where flash roastin is taking place.

4. Provision should be made for controlling the temperature .at which roasting proceeds.

5. The roasting operation should be conducted so that suitable auxiliary reagents (e. g. sodium chloride) may be introduced into the charge at optimum points, when such is desired.

6. The roasting apparatus should be simple, of rugged construction, utilizin known and tried components to the maximum extent possible.

7. Replacement or repair of roaster parts subject to most rapid wear or damage should be accomplished quickly and easily; and to the greatest extent possible without shutting down andcooling the furnace.

The improved metallurgical roasting furnace we have designed on the basis of these criteria is formed with a cylindrical wall and has therein a plurality of vertically spaced hearths. Each roasting hearth, however, instead of being of the circular arched refractory design common to multiple hearth masters as heretofore known, comprises several substantially sector-shaped hearth segments (or pallets) arranged in a common plane, with'the side edges of each segment spaced laterally from the adjacent side edges of the neighboring hearth segments in the same plane. Thus a plurality of substantially radial drop holes are provided in each hearth, through which material on any hearth segment may be showered to the hearth below. The hearth segments in any one plane, or hearth level, are oil'- set laterally with respect to the segments in the plane next above, to the extent necessary that material falling vertically through the drop holes will be caught on a hearth segment of the next lower hearth.

Each hearth is provided with rabble arms carrying rabbles that may be set radially of the furnace to push the charge on each hearth segment primarily in a circumferential direction toward the drop hole; or that may be set at an angle (which alternates in direction of pitch with without any refractory protective insulation, and' are hollow to permit circulation therethrough of a cooling fluid. Each sector-shaped hearth segment is formed with a mounting flange at its outer arcuate or circumferential edge for fastening it to the cylindrical furnace wall. As this is the only support that need be provided for the hearth segment, it may be withdrawn through, the furnace wall and a .replacement may be substituted without it being necessary for workmen to enter the interior of the furnace, or even to enables to be carried out involves causing a charge of finely divided ore or concentrate, heated to the roasting temperature, to fallfreely in .the form of a thin elongated stream through 4 y the oxidizing furnace atmosphere. The density of the falling paritcles in the stream is kept low enough so that every particle is in contact with the oxidizing atmosphere. This low particle density is more or less characteristic of the best practice in flash roasting, but, contrary to flash roasting, the roasting carried outv by the above described apparatus further involves interrupting the free fall of the particles after they have descended only a small fraction of the height of the roasting chamber. Such interruption occurs, of course, by catching the fallin particles on the hearth segment below the drop hole through which they have fallen, and here they are held long enough to further the roasting of the coarser particles. Thereafter the material is again caused to fall another small fraction of the height of the roasting chamber, and again the fall is Usually the duration of the interruption is not more than about one-quarter hour. In normal operations, some part of the charge is at almost all instants undergoing flash roasting while the remainder is undergoing bed roasting on the hearth segments, and all parts of the charge undergo flash roasting and bed roasting alternately.

In a preferred embodiment of the invention, a.

stream of air (or oxygen, or oxygen-enriched air) is directed laterally against the charge as it begins to fall from one hearth level to the next. In this manner an ample supply of air is provided during the period of flash roasting, when the roasting process is proceeding most intensely. The air stream also can be made to result in some size classification of the charge particles, by delivering it with sufficient force to blow the finest particles out of the path of substantially vertical fall traversed by the coarsest particles. The force of the air streammay be so controlled that the total duration of the interruptions in the fall of the flnestparticles as they traverse the roaster is substantially less than the total duration of the interruptions in the .fall of the coarsest particles as they traverse the chamber. In this manner the line particles which roast to completion most rapidly, pass through the furnace in a shorter period of time than the coarse and hence more slowly roasted particles. J

Control of the roasting temperature is eflected by controlling the duration of the interruptions between falls of the charge particles (i. e. by controlling the number and speed of rotation of the rabble arms), and by controlling the extent of cooling of the hearth segments. Additional control over the roasting temperature is also effected by the conventional expedient of controlling the amount of air admitted to support combustion of the roasting charge.

The foregoing is merely an outline of some of the major features of the invention. These and other features are described in greater detail below with reference to the accompanying drawings, which show a roasting furnace designed in accordance with the invention.

In the drawings,

Fig. 1 is a perspective, with parts broken away, showing the hearth levels of a furnace according to the invention in which interrupted flash roasting is proceeding;

Fig. 2 is an elevation of a multiple hearth furnace according to the invention;

Fig. 3 is a plan of the furnace shown in Fig. 4 is a vertical section through the furnace shown in Fig. 2;

Fig. 5 is a horizontal section through the furnace of Fig. 4, taken substantially along the line 55 of Fig. 4;

Fig. 6 is a horizontal section through a hearth segment for the roasting section of the new furnace, taken substantially along the line 6-8 of Fig. 7;

Fig. '7 is a cross section taken substantially along the line 1-1 of Fig. 6; and

Fig. 8 is a vertical section taken substantially along the line 88 of Fig. 6.

method which the new apparatus makes possible.

Apparatus The general arrangement of the new roasting furnace in the region wherein interrupted flash roasting takes place is best shown in Fig. 1. The furnace comprises a. cylindrical steel shell I having a refractory lining II and an axial rotatable hollow metallic column I2 provided with a refractory facing i3. A series of hearths at vertically spaced hearth levels indicated by the arrows A, B and C are provided within the furna-ce wall. The hearth at each level A, B, etc. comprises a series of sector-shaped hearth segments l4. Hearth segments indicated by reference numerals HA, HA, etc., are arranged in the common plane of the hearth level A, hearth segments indicated by reference numerals NB, NB, etc., are arranged in the common plane of the next lower hearth level B, etc. The radial side edges l (herein called the forward side edges) of each hearth segment are spaced laterally from theadjacent radial side edges I 6 (the rearward" side edges) of the neighboring hearth segments in the same common plane or hearth level. Thus between each adjacent pair of hearth segments in a given hearth level is a fairly wide radial opening (such, for example, as the opening between the forward side edge I5 of hearth segment HA and the rearward side edge l6 of the hearth segment HA) which serves as a drop hole through which are or concentrate being roasted may fall from one hearth level to that next below.

It will be noted that the hearth segments in any given hearth level are staggered laterally with respect to the hearth segments in the levels next above and next below. For example, the hearth segments MB, NB, etc., in the B hearth level are not arranged directly below the hearth segments MA, MA, etc., in the A hearth level next above, but are offset laterally with respect thereto so as to underlie the radial drop holes between these latter hearth segments. Thus each hearth segment is in position to receive material falling vertically over the forward edge ii of a hearth segment in the level next above it.

Rabble arms I! project laterally from the axial column 12 at each hearth level. Each rabble arm carries a series of rabbles I8. The rabbles, contrary to usual multiple hearth furnace practice, are shown in Fig. l arranged parallel to the radially extending rabble arms, and they serve to push ore or concentrate being roasted on any of the hearth segments circumferentially toward the forward side edge [5, over which it may fall through the wide radial drop hole to the hearth below. In view of the arrangement and action of the rabbles l8, they might more accurately be termed pushers, but we prefer to use the more conventional term rabbles to identify them. Therabbles l8 may, however, be set at an angle to the axis of the rabble arm. If this is done the pitch of the angle should alternate from one rabble arm to the next, so that while the rabbles then will keep turning over the charge on each hearth segment, they will not, in net effect, tend to move it radially in or out along the hearth, but will move it circumferentially to the drop hole.

As shown in Figs. 1, 6, 7 and 8, each hearth segment i4 is a hollow metallic sector-shaped element having top and bottom walls l9 and 20 joined together by the Side edges l5 and 16. Each such segment is divided interiorly by a partition 2| into two compartments 22 and 23. The partition 2| does not extend all the way to the apex end 24 of the hearth segment, but instead treminates at a point 25 short of the apex, so as to provide an opening 26 for communication betweenthe lower and upper compartments 22 and 23 within the segment.

The arcuate cr circumferential end portion 21 of each hearth segment is enlarged to provide cooling fluid inlet and exhaust passageways 28 and 29. The inlet passageway 28 communicates with the compartment 22 below 'the partition 2|, and the exhaust passageway 29 communicates with the compartment 23 above the partition. The inlet and exhaust passageways advantageously are formed integrally with the hearth segment, and integrally also with mounting flanges 30 by which the segment is fastened in place in the furnace. Each hearth segment extends through a circumferentially-extending slot in the steel shell in of the furnace, and is secured in place by bolting the mounting flanges 30 to the furnace shell. This is the only support necessary to retain the hearth segment in place, and to enable it to carry its own weight and that of the charge which falls upon it. Consequently it is a simple matter to remove or insert any particular hearth segment, without entering the interior of the furnace or even discontinuing its operation.- To facilitate inserting a hearth segment into the furnace, or removing it therefrom, the top and bottom wal's of the cooling fluid inlet and exhaust passageways are tapered as indicated at 3| (Fig. 8).

To keep the hearth segments from overheating during operation of the furnace, cooling air (or other cooling fluid) is admitted to the in terior of each segment through a supply pipe 32 communicating with the inlet passageway 28. The air flows thence through the lower compartment 22 to the opening 26 and back through the upper compartment 23 to the exhaust passageway 29, from which it escapes through an exhaust pipe 33. The longitudinal partition 2| thus serves primarily as a baffle for directing the cooling air to the apex 24 of the segment before it escapes. In place of using a horizontal longitudinal partition 2| (as shown in the drawings) for this purpose, a vertical longitudinal partition can be used instead.

A vertical longitudinal partition 34 (Figs. 3

' the third hearth of the furnace).

air manifold 35 or openings 36.

hearth segment near its forward side edge IE, to

1 form a combustion air inlet manifold 35. The

forward side edge I5 is itself formed with a series of openings 36, which serve the purpose of dimanifold 35 through inlet pipe 31 controlled by a valve 38 (Fig. 6), and may be taken with advantage, as preheated air, from the exhaust pipe 33 through which spent cooling air escapes from the outlet passageway 29. If some other medium than air is used for cooling the hearth segments, the valved inlet pipe 31 is connected to some other convenient source of air or other oxidizing gas.

A substantially complete roasting furnace, in which interrupted flash roasting hearths according to Fig. 1 are incorporated is shown in Figs. 2 to 5. The furnace is shown as having six vertically spaced hearths 40 to 45, but additional hearths may be provided if desired. The uppermost hearth 43 receives the charge and serves as a preliminary drying hearth. The second hearth 4| is a drying and distributing hearth,"

and is arranged to insure proper distribution of the charge on the hearth segments of the first interrupted flash roasting hearth 42 (shown as Additional interrupted flash roasting hearths, in any desired number, are arranged below in the manner describedabove with reference to Fig. 1 (only two such additional hearths43 and 44 are shown in Fig. 4). The lowermost hearth45, which forms the floor of the furnace, is a finishing hearth from which the roasted product is with drawn.

The two upper drying hearths 46 and 4| are full circular hearths. They may be of the usual arched refractory brick construction, but advantageously they are made .of metallic segments of substantially the same construction shown in Figs. 6 to 8, but without the longitudinal partition 34 as there is here no need for a combustion The upper drying hearths may be heated rather than cooled to facilitate drying of the charge.

Ground ore or concentrate is delivered to the upper drying hearth 46 and is spread evenly on it by a charge distributor 46 I (Fig. 4). The charge distributor receives the material to be roasted from' a hopper 41 to which it is delivered through a feed pipe 48. A screw conveyor 49 inside the charge distributor moves charge radially out from the hopper 41, discharging it through distributing outlets 50 along the radius of the hearth. At the same, time the distributor is moved slowly in a circular path about the axis of the central column, thus spreading the charge,

cover in their circular path, and also operates the worm conveyor 49. A seal plate 55 extending up from the furnace shell provides a fairly gas-tight joint between the cover 5| and the main body of the furnace.

The upper hearth 40 is provided with a series of radial drop holes 56 (Fig. 5) and is served by rabbles 51, 51 carried on rabble arms 58, 58'

direction in which the rabbles 51 on one arm 58 are pitched is opposite to that of the rabbles 51' on the other arm 58'. The net result of this arrangement is that the rabbles do not move the charge substantiallyeither in or out along the radius of the furnace, but keep turning it over and working it circumferentially toward the drop holes.

The second drying hearth 4| is of essentially the same construction as the upper drying hearth 40. It is provided with radial drop holes 59, and is served by rabbles 60, 60' arranged in the same way on rabble arms 6|, 6| as the rabbles 51, 51 serving the upper hearth. However, the radial drop holes 59 of the second hearth are located so as not to lie directly underthe drop holes 56 of the upper hearth (otherwise charge falling from the upper hearth 40 would not be caught on the second hearth 4|).

The next three hearths 42, 43 and 44 shown in Fig. 4 are interrupted flash roasting hearths constructed and arranged as described above in connection with Fig. l. The hearth segments of the uppermostof these hearths 42 are so locatedthat their rearward edge portions underlie the radial drop holes 59 of the second drying hearth, so as to catch the charge as it falls from the lowermost drying hearth. The interrupted flash roasting hearths 42 to 44 are served by rabbles |8 carried on rabble arms l1, as previously described, and these rabbles may if desired be set parallel to the rabble arm so as to act-more like pushers for moving charge directly across the hearth segments to the radial drop holes betweensegments. However, the roasting hearth rabbles I8 may alternatively be arranged in the same manner as the rabbles 51, 51' and 60, 60' serving the drying hearths, so as to move the charge more slowly across the roasting hearth segments and turn it over more effectively.

The lowermost hearth 45 is in effect the bottom of the furnace, and is advantageously of refractory brick construction. It is served by rabble arms 62 carrying rabbles 63 which are set at an angle to move the charge outwardly from the center of the furnace to the periphery, in the conventional manner of "out hearths of ordinary multiple hearth furnaces. The roasted product is discharged from the bottom hearth 45 through one or more outlets 64 at furnace periphery.

The central column 2 on which the rabble arms I1, 58, BI and 62 are mounted is of more or less conventional construction. It is advantageously in the form of a large hollow metallic shaft, provided with hollow rabble arm socket rings 65 at each hearth level. The column is supported on a heavy roller bearing 66 at its base, and is provided with the usual ring gear 61, through which the column may be slowly rotated by a pinion 68 and associated drive mechanism. A bearing 69 mounted in a supporting frame 1|! (which may in turn be supported from the furnace shell) is provided near the top of the column I! to hold it upright in its axial position within the furnace.

The arrangement for cooling therabble arms is conventional, but is described below in' some detail for the sake of clarity. A cooling fluid inlet pipe 1| communicates with the interior of the'column through the central opening of the supporting bearing 66. The interior of the colpipe.

umn is divided into two compartments I2 and I3 by a longitudinal partition 14. Cross partitions at the top and bottom of the longitudinal partition I4 prevent direct communication between the compartments l2 and 13 on opposite sides of the longitudinal partition. Cooling air delivered through the inlet conduit 1| enters only the compartment 12 on one side of the partition I4.

Each of the rabble arms'l'l etc. below the upper cross partition I5 is hollow and is provided interiorly with a pipe 16 or 16' which extends from the inner end of the rabble arm almost to its outer end. Pipes "I6 in rabble arms mounted on one side of the partition 14 communicate with one of the compartments l2, and pipes 16' in rabble arms on the opposite side of the partition communicate with the other compartment 13. Each annular rabble arm socket ring 85 is hollow, defining an annular cooling medium passage 17, and the interiors of the rabble arms l1 etc., outside the pipes 16, communicate with this passage through ports 18 and I8. I

Cooling air or other cooling medium entering the compartment 72 through the inlet conduit 'H can flow only into the pipes 16, and then from the ends of these pipes must flow back through the hollow rabble arms and through the ports 18 into the annular passages 11 in the rabble arm socket rings. The air then flows through these passages to and through the ports 18' and into the interiors of the rabble arms mounted on the opposite side of the partition 14. Thence the fiow is through the rabble arms and back through the pipes 16 into the compartment 13.

The spent cooling air passes from the compartment I3 throughan exhaust conduit 'l9 at the top of the furnace. A liquid cup seal 80 is provided at the junction of the exhaust conduit 19 with the axial column I 2. A damper may be provided in this conduit near the top of the furnace, if desired, to control the rate of flow of cooling air through the rabble arms.

In order to prevent overheating of the metallic hearth segments and to provide for control over the temperature prevailing during roasting, a cooling fluid supply header 8| (Figs. 2 and'3) connected to bustle pipes 82 is arranged to deliver cooling fluid to each of the hearth segments of the interrupted flash roasting hearths 42, 43, 44, etc., and to the lowermost drying hearth 4| (since the bottom of this hearth is exposed to the roasting zone of the furnace, it requires effective cooling). The supply pipes 32 of the hearth segments are connected to this bustle The cooling fluid entering each hearth segment flows through the compartment 22 on one side of the partition 2| and back through the compartment 23 on the other side to the cooling fluid exhaust passageway 29 (as above described in connection with Figs. 6 to 8) whence it flows through the exhaust pipes 33 to a second set of bustle pipes 83- connected to an exhaust head- If desired, provision may be made to utilize the heated spent cooling air from the interrupted flash roasting hearth segments for preheating and drying the incomingcharge on one or more of the upper drying hearths 40. In the arrangement shown in Fig. 2 for this purpose, a pair of bustle pipes 85 and 86 are connected to the exhaust header 84 below and above a damper 81,

respectively. The supply and exhaust pipes 32 and 33 of the hearth segments are connected to these bustle pipes (here the pipes 32 that are exhaust pipes on the roaster hearth segments are used as supply pipes, and vice versa, for effective heating and drying of the charge). Any desired proportion of the hot spent cooling air from the roasting section of the furnace may be by-passed from the exhaust header through the segments of the drying hearth 40 by appropriate adjustment of the damper 81.

To provide for withdrawal of roaster combustion gases, one or more combustion gas outlet flues 88 (preferably at least one such flue serves each hearth) connects the interior of the furnace with a combustion gas exhaust stack 89. Dampers 90 in the flues 88 are provided for regulating the draft through the furnace. A conventional gate valve 90 at the bottom of the stack is provided for removal of flue dust that settles out in the stack.

At least one access door-9| is provided at each hearth level, both. to permit observation of and access to theinterior of the furnace, and to admit combustion air. Any desired number of fuel burners 92 may also be provided to heat the charge to the roasting temperature when starting the furnace, and also to keep the charge at the roasting temperature if it does .not contain enough sulphur to roast autogenously.

Downwardly sloping annular protective shields 83 (Fig. 4) are secured to the rotatable central column I2 at each hearth level to prevent particles of charge from falling between the refractory facing [3 of the rotating central column and the adjacent edges of the stationary hearth segments. The hearth segments may be provided at their inner ends with an upstanding lip 84 which extends up behind the cooperating shield 83.

A feature of importance of the new furnace,

' resulting from the use of all-metallic sectorshaped hearth segments fastened only to the furnace shell is the ease with which a hearth segment may be removed for repairs and maybe replaced without shutting down the furnace. The hearth segment is simply withdrawn radially through the circumferentially extending slot in the furnace wall in which it is mounted, as indicated in dotted lines at l4, Fig. 3. Each of the hearth segments normally is held in place solely by bolts 95 passing through holes 86 in its mounting flanges 30 and threaded into tapped holes in the steel shell ll! of the furnace. No fastening means are needed inside the furnace for holding the hearth segment in place. The flange mounting is adequate for'supporting both the weight ofthe hearth segment itself and any charge upon it. Accordingly, by simply removing the bolts 95, the entire hearth segment may be withdrawn from the furnace and another similar hearth segment may be inserted and fastened in place. If there is an adequate draft through the stack 89, this operation may be performed even while roasting is going on within the furnace.

' Method I Roasting is carried out in the furnace described above substantially as follows: Quite finely 'crushed ore, or a flotation concentrate of the usual degree of fineness, such as a zinc sulphide or copper sulphide mineral, is introduced through the feed pipe 48 and is spread evenly over the 11 drop holes 59, through which it falls to the hearth segments (or pallets) of the first roasting hearth quite uniformly along the radial length of the underlying segments of the first interrupted flash roasting hearth 42. As it falls thereto, ity

is heated by the hot atmosphere in theroasting section of the furnace to the temperature at which roasting begins. Assuming the charge to 1 be a sulphide mineral capable of autogenous roasting-i. e. able to sustain combustion-some roasting will be initiated as the charge falls from the last drying hearth, and the roasting reaction will spread through the charge as it collects on the segments of the first interrupted flash roasting hearth.

Quite shortly after a portion of the charge has been deposited on a segment of the first interrupted flash roasting hearth, it will be pushed therefrom over the forward side edge by the 3 action of the rabbles l8, and flash roasting will ensue as it showers to a segment of the next lower hearth level. Bed roasting then proceeds for a further short period of time, until the charge is again pushed by the rabbles over the forward side edge of the hearth segment, to fall in a flash roasting environment to the next hearth. These periods of flash roasting with intervening periods of bed roasting on the hearth segments follow one another until the charge finally arrives at the bottom hearth 45 of the furnace. Here the rabbles move the charge to the outer periphery of the furnace, where it drops out through the discharge conduits B4.

The combustion gases formed during roasting of the charge pass out through theflues 88 and escape through the stack 89. With one or more flues 88 serving each hearth level, the nature and extent'of the draft through the furnace is readily '90 in each flue. v

The movement of the charge through the roasting section of the furnace is most clearly visualized by reference to Fig. 1. Once the charge has been distributed uniformly along the radial length of the hearth segments in the first roasting hearth level (say hearth level A), it is moved quite uniformly by the rabbles over the forward side edges of the segments and falls in a thin 1 elongated stream of low particle density to the segments of the hearth next below. It is de* sirable of course that the particle density in the stream of charge falling from one hearth to the next be sufficiently small so that every particle is in contact with the oxidizing atmosphere of 1 controlled by suitable regulation of thedamper l the furnace during its fall. This is necessary to insure optimum flash roasting. Conventional 1 flash roasting furnaces are, of course, designed to shower th charge through the roasting chamber in a stream of low particle density; but in theconventional multiple hearth furnace, the particle density of the stream of charge falling through the drop holes is so great that only very incomplete flash roasting can take placethe high particle density of the falling charge in such furnaces locally depletes the oxidizing constituents of the furnace atmosphere to such an extent that many of the particles fall all orpart of the distance to the next hearth below without coming in contact with suflicient oxidizing gas to be effectively roasted. 7

Two results of major importance ensue from the above-described method of interrupted flash roasting. The first such result is that the total time of flash roasting, in a furnace of given height, is very substantially increased as compared with a conventional I flash roasting operation. This follows from the fact that as a particle falls itv accelerates, and traverses a given distance much more rapidly toward the end of its fall than toward. the beginning. For example, a freely falling body takes 1.12 seconds to fall vertically 20 feet;'but if the fall is interrupted at, say, 4-foot=intervals, the falling time to traverse a total distance of 20 feet is increased to 2.5 seconds. This is precisely what occurs in the furnaces of this invention--neglecting the hindering effect of an updraft through the furnace, the time of free fall for each particle of the charge traversing the roastingsection of the furnace is substantially greater than in a conventional flash roasting operation in a roasting chamber of equal height. At the same time the low particle density in the flowing stream of charge that is characteristic of flash roasting but not of conventional multiple hearth roasting is achieved. r

The second major result is that during the periods the charge is held on the segments of the several hearths, the. larger particles are given particle may be roasted completely in the short period of time allowed. In the interrupted flash roasting carried out in the new furnace, however, substantially larger particles in the charge are permissible, because the time during which the particles are held in a roasting environment on the hearth segments is very much greater than can be achievedin ordinary flash roasting. On the other hand, the time of retention 'on the hearth segments is much shorter than inconventional multiple hearth roasting, so that the charge may be advanced much more rapidly through the furnace.

With rabbles l8 set parallel to the rabble arm l1 and acting as pushers, the'time of retention of the charge at each hearth level is determined by the number of rabble arms and by the speed of rotation of the central shaft. An increase in the number of rabble arms at any given hearth level for a given speed of rotation of the shaft decreases the time interval between two successive passes of the rabbles over a given hearth segment; and, of course, such time interval decreases with an increase in the speed of rotation of the central column. Since pusher type rabbles wipe a hearth segment substantially clean each time they pass over it, the retention time can be controlled by the numberof rabble arms used and their speed of rotation. The same is for themost part true if the rabbles are inclined at an angle to the rabble arms, except that in this case 13 of time that is long in relation to the length of time required for free fall through the height of the furnace, but which, in general, does not exceed about one-quarter hour. A quarter-hour is a considerably shorter time interval than thatduring which the charge is held on each hearth of a conventional multiple hearth roaster (which is generally about an hour or so). It is a period of time that is ample for effective roasting of charge particles much coarser than can be treated successfully in conventional flash roasting operations; and yet it is so short in relation to the time interval involved in multiple hearth roasting as to greatly increase the rate at which a charge may be roasted.

As indicated above, it is very desirable to direct a stream of air (or other oxidizing gas) laterally against the charge, as it falls from one hearth level to the next. In the apparatus shown 'in Fig. 1, a stream of air is delivered into contact with the charge through the openings 36 as it falls over the forward side edges 15 of each hearth segment. Several advantageous results are accomplished in this manner. One result is that an ample supply of oxidizing gas is brought into contact with the charge at just the point where it is required-that is, at the point where flash roasting commences or resumes. A second result is that the stream of air, if delivered with suflicient force, blows the finer particles of the falling charge farther out from the forward edge of the hearth segment than the coarser particles, thus forcing a decreased particle density in the falling stream of charge. In consequence of this second result, the finer particles fall to the hearth segment next below at a point nearer its forward edge than do the coarser particles. Thus these finer particles, which require a minimum of bed roasting, are in position to be among the first to be rabbled to the next drop hole. If inclined rather than pusher type rabbles are used, the fines falling toward the forward edge of the hearth segment may be rabbled to the drop hole with the very next pass of a rabble arm, while the coarser particles may not be advanced to the drop hole until after several passes of a rabble arm over the hearth segment. Still greater size classification of the v particles results if the stream of air issues from the openings 36 with sufficient force to blow the finest particles in the charge far enough forward to miss the hearth segment immediately below, and to continue falling at least to the second hearth below.

To illustrate the foregoing, consider a charge falling over the forward edge l of a hearth segment MA in the A hearth level of Fig. 1. Should the charge fall vertically, it would be caught near the rearward edge of a hearth segment MB in the B hearth level. However, a stream of air issuing from the openings 36 in the upper hearth segment MA will blow some of the fines forward to be caught near the forward edge of the lower hearth segment MB; and if the force of the air stream is sufficiently great, only the coarser particles will be caught at all on this hearth segment MB, while the finest particles will be blown forward sufficiently to fall through the radial drop hole between the hearth segments MB and MB" in the B hearth level to the hearth segment MC two hearth levels below. Thus in this latter case the finest particles miss the B hearth level a1together-their fall is interrupted at the C hearth level, substantially lower in the furnace than the B hearth level where the fall of the coarse particles is interrupted. In either quickly, may thus be made to traverse the furnace in a substantially shorter length of time than the coarser particles, which require a longer time for roasting.

Effective control of temperature is important in order to secure most satisfactory roasting results. It is virtually impossible to control the temperature of a conventional flash roasting operation, particularly at different levels in the flash roasting zone. Considerably better success in thisrespect is attainable in multiple hearth roasting operations, but often at the expense of decreasing the capacity of the roasting furnace. This is because-the temperature control in such furnaces is generally achieved by controlling the rate of speed of the rabble arms and the rate at which air is admitted to the furnace. Temperature control in these manners directly afiects the rate at which the charge is or can-be passed through the furnace. Furthermore, the high reaction temperature attainable in properly controlled flash roasting cannot be reached in conventional multiple hearth roasting.

In the roasting operation carried out in the furnace of this invention, the same temperature controls used in ordinary multiple hearth roasting are available, but in addition the temperature may further be controlled by regulating the flow of cooling air or other cooling medium through the metallic hearth segments. Temperature control in this latter manner is often more precise than by the methods available in ordinary multiple hearth roasting operations, and in addition permits different temperatures to be maintained, within limits, at different hearth levels in the roasting section of the furnace. For example, in roasting an iron-bearin zinc sulphide concentrate, it is generally desirable to'avoid the formation of complex zinc-iron compounds. Such compounds will form if zinc sulphide is in contact with iron'sulphide in a bed of charge heated to the roasting temperature of zinc sulphide. However, if the roasting operation is initiated at a relatively low temperature, at which the iron sulphide oxidizes readily but at which the zinc sulphide is not significantly affected, then subsequently the zinc sulphide may be roasted at the higher temperature required for its oxidation without formation of undesirable complexes. The roasting apparatus of the invention permits of achieving such preferential initial roasting of one component of a charge, before roasting of the other component begins to any substantial extent, because sufliciently close control of the temperature of the roasting charge at each of the different hearth levels may be attained.

The rate at which flash roasting proceeds relative to the rate of bed roasting also may be controlled within limits by controlling the rate at which air is admitted through the openings 36 at the forward edges of the roasting hearth seg- 'ments. This serves also as an added means for controlling the temperature in the furnace, for the relatively high temperature developed in flash roasting is transferred by the furnace atmosphere to all parts of the furnace.

Roasting operations sometimes are carried out in conjunction with an added solid reagent, as for example when sodium chloride is added to a roasting charge to effect a chloridizing roast. It is generally not feasible to incorporate an added reagent in the charge delivered to a suspension roasting operation, because the time interval of the roasting operation is too short for a plurality of diflerent reactions, one dependent upon the other, to proceed to substantial completion. When using the furnace of the invention, however, it is thoroughly feasible to incorporate a solid reagent with the charge, either when the charge is first delivered to the furnace.

or at some point in the roasting section of the furnace below that at which roasting first begins, in the same manner as is possible in ordinary multiple hearth roasting. The reactions involving the added reagent usually proceed more provide substantially radial drop holes therebetween, a conduit having apertures adjacent a side edge of each of said segments, and means for delivering a current of fresh air through the apertures of said conduit into contact with material being roasted as it falls through said drop holes.

2. In a metallurgical roasting furnace of the character described, a roasting hearth comprising several substantially sector-shaped hearth segments arranged in a common plane with the side edges of adjacent segments spaced apart to provide substantiallyradial drop holes therebetween, rabbles for moving material being roasted across said hearth segments to the drop holes,

. and an apertured conduit positioned for directing a current of an oxidizing gas from the side edges of the hearth segments horizontally against the material being roasted as it falls vertically through said drop holes.

3. Ina metallurgical roasting furnace of the character described a roasting hearth comprising several hollow metallic substantially sectorshaped hearth segments arranged in a common plane with the side edges of adjacent segments spaced apart to provide substantially radial drop holes therebetween, an inlet conduit for introfor withdrawing warm spent cooling air therefrom, a combustion air inlet manifold along one a side edge of each segment, the sides of said segments adjacent said manifold being formed with openings through which air may pass from the manifold into contact with material being roasted as it falls through said drop holes, and a pipe connecting the cooling air exhaust conduit with said manifold, whereby .warm spent cooling air may be delivered as combustion air into contact with the roasting charge.

4. In a roasting furnace having a cylindrical shell and a rotatable axial column on which rabble arms are mounted, a roasting hearth comprising several individual metallic fluid-cooled sector-shaped hearth segments, each segment having a mounting flange at its arcuate edge, and each segment being mounted within the furnace between the central column and the furnace shell and being supported solely by attachment of its 16 mounting flange to the outer surface of the furnace shell.

5. ma roasting furnace of the character described having a cylindrical shell, .an individual hearth segment comprising a hollow sectorshaped metallic element having a longitudinal partition dividing the interior thereof into at least two compartments, said partition having an opening therein adjacent the apex of the se ment providing for communicationbetween said compartments, a cooling medium inlet conduit connected with one of said compartments and a cooling medium exhaust conduit connected with the other of said compartments, and a mounting flange at the arcuate edge of said element for attaching the segment to the outer surface of the cylindrical furnace shell.

6. A metallurgical roasting furnace of the character described having an upper drying section including a drying hearth and a lower roasting section including a plurality of roasting hearths, each of said hearths being composed of hollow metallic elements, a cooling medium supply conduit connected to said roasting hearths for delivering a cooling medium to the spaces within the hollow roasting hearths, a cooling medium exhaust conduit connecting said spaces within the roasting hearths to the space within the hollow drying hearth, whereby spent heated cooling medium from the roasting hearths of the roasting section may be delivered to the drying hearth of the drying section in heat exchange contact with a charge to be dried thereon, means for introducing combustion air into the roasting section, and means for exhausting the products of combustion therefrom independently of the spen heated cooling medium.

7. In a metallurgical roasting furnace of the character described, a roasting hearth having at least one drop hole, a conduit having apertures adjacent the side edge ofthe drop hole, and means for delivering a current of air through the apertures of said conduit into contact with ducing fresh cooling air into the interior of the hollow hearth segments and an exhaust conduit material being roasted as it fallathrough said drop hole.

8. A metallurgical roasting furnace as set forth in claim 7. in which the roasting hearth is circular, the drop hole is radial in extent and the conduit is located adjacent one side edge of the radial drop hole.

- ERNEST KLEPETKO.

, PHILIP in: B. KAYE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA Germany Sept. 21, 1936 

